This is the exact opposite of your question, but may be a good use to clean and start your aging. Copper when heated to red and then dunked into isoprophyl alcohol will be as bright as polished. This removes all the surface junk.
I use "Iso-Heet" for this. Do use in an well vented area. I have never had the alcohol flame.
The first space shuttle had some copper parts my partner had welded up and his was how he cleaned those parts.